1. Field of the Invention
The present invention relates to a front derailleur for effecting a speed change operation with respect to front gears of a bicycle with a front three step speed change mechanism comprising a small, an intermediate and a large gears, and more particularly to a front derailleur provided with a first pressing portion having an arc substantially coaxial with the front gears and including a first pressing point where a pressing operation of the chain engaging the small gear is started and a second pressing point where the shifting operation of the chain mounting the intermediate gear is completed when a first speed change operation is effected for shifting the chain from the small gear to the intermediate gear, the first pressing portion passing through the first and second pressing points, and with a second pressing portion positioned radially outwardly of the front gears relative to the first pressing portion for pressing the chain when a second speed change operation is effected for shifting the chain from the intermediate gear to the large gear.
2. Description of the Prior Art
An example of the conventional front derailleurs of this type is shown in FIG. 11. This derailleur comprises a derailleur body 2 including an inner guide plate 600 acting for mainly shifting the chain from the smaller gear to the larger gear and an outer guide plate 500 acting for shifting the chain from the larger gear to the smaller gear. The derailleur body 200 is parallel to the front gears G and moves perpendicularly to the gear through a four-point link mechanism 400 attached to a seat pipe 1 to effect a speed change operation. Since a mountain type bicycle or the like has recently been come into fashion, such front gears G for a front three step speed change mechanism are used for improving a traveling efficiency when traveling on a mountain road in a low or high torque as include a small gear GS having a considerably decreased diameter compared with an intermediate gear GM and a large gear GB and the intermediate and large gears having substantially the same diameters as the conventional gears. The front derailleur is manufactured to be suitable for such front gears G, which is characterized by the inner guide plate 600 having a larger vertical dimension than the conventional structure. An initial pressing point (referred as a first pressing point 700-A FIGS. 11 and 12) where the chain C engaging the small gear GS is pressed to be shifted to the intermediate gear GM is in a lower position compared with the conventional art. More particularly, in order to complete the shifting operation to the intermediate gear GM while pressing the chain C mounting the intermediate gear GM at a first pressing portion 700 profiling an arc from the first pressing point A substantially coaxially with the gears, a lower portion of the first pressing portion 700 is also positioned in a lower level than the conventional derailleur. With this structure, when a first speed change operation is effected for shifting the chain from the small gear GS to the intermediate gear GM, only the first pressing portion 700 contributes to press the chain C to complete the shifting operation.
When a second speed change operation is effected for shifting the chain C from the intermediate gear GM to the large gear GB, the speed change operation is initiated at a point positioned on the first pressing portion 700. The pressing point of the chain C moves upwardly toward a forward end of the first pressing portion 700 with advance of the speed change operation. As the pressing point moves, the chain is also pressed at a second pressing portion 800 positioned radially outwardly of the first pressing portion 700. FIG. 12 shows a positional relationship between the first pressing portion 700 and the second pressing portion 800 on the inner guide plate 600-a. The second pressing portion 800 around an extreme end thereof is substantially above the first pressing portion 700, and these pressing portions are formed of a plate continued in substantially the same plane. As a result, when the speed change operation further advances and the chain C mounts the large gear GB to complete the shifting operation, the chain C is pressed almost only by the second pressing portion 800.
With the above noted structure for effecting the speed change operation from the smaller gear to the larger gear, the speed change operation from the small gear GS to the intermediate gear GM can be advantageously effected by an extension 600a' provided downwardly of the inner guide plate 600. However, provision of the downward extension produces a problem when the chain C is shifted from the intermediate gear GM to the large gear GB. As seen from FIG. 11, the inner guide plate 600 overlies a lateral side portion GBa of the large gear GB when viewing from a bicycle body. Additionally, the chain C is pressed at a lateral side thereof by a portion of the plate between the first pressing portion 700 and the second pressing portion 800 in the course of shifting from the intermediate gear GM to the large gear GB. Thus, the chain C tends to extend parallel to the gear lateral side portion GBa and falls into a space between the inner guide plate 600 and the lateral side portion GBa of the large gear as the speed change operation proceeds, which results in bite or strain of the chain or the like to deteriorate an operational efficiency at the time of the second speed change operation.